Tehran, which insists its intentions are peaceful, is negotiating with the so-called P5+1 group comprising the five permanent members of the UN Security Council - the US, Russia, China, the UK and France - plus Germany.

No new round of talks was scheduled after two days of negotiations in Almaty, Kazakhstan, which ended on Saturday.

'Back on track'

After recently helping broker a reconciliation between Turkey and Israel, Mr Kerry said: "We would like to see this relationship that is important to stability in the Middle East and critical to the peace process... get back on track in its full measure.''

That, he told reporters in Istanbul, meant promises of "compensation be fulfilled, ambassadors be returned and full relations be embraced".

Mr Davutoglu said Turkey would be "careful" in its approach, and stressed that Israeli trade restrictions on the Gaza Strip "should be eliminated once and for all".

Mr Kerry also met Turkish Prime Minister Recep Tayyip Erdogan.

More than 1.2 million people have fled to neighbouring countries since the unrest in Syria began two years ago, according to UN figures.

At least 250,000 are in Turkey.

Both Turkey and the US oppose Syrian President Bashar al-Assad, with Turkey holding that the West should do more to help the Syrian opposition.

Afghan deaths

The start of Mr Kerry's trip was overshadowed by the death of six Americans, along with an Afghan doctor, in two attacks in Afghanistan.

The Americans included a foreign service officer who had acted as Mr Kerry's guide during his recent visit to Kabul.

Anne Smedinghoff, 25, had been delivering books to students in southern Zabul province.

She was the first US diplomat killed in service since last year's attacks in the Libyan city of Benghazi.

Speaking to US consulate workers in Istanbul, Mr Kerry said Ms Smedinghoff's death was a "grim reminder... of how important, but also how risky, carrying the future is".